Differential affinities of visual arrestin, beta arrestin1, and beta arrestin2 for G protein-coupled receptors delineate two major classes of receptors

Reciprocal regulation of angiotensin receptor-activated extracellular signal-regulated kinases by beta-arrestins 1 and 2

beta-Arrestin2 not only plays essential roles in seven membrane-spanning receptor desensitization and internalization but also functions as a signal transducer in mitogen-activated protein kinase cascades. Here we show that the angiotensin II type 1A receptor-mediated activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) in HEK-293 cells is increased when the cellular level of beta-arrestin1 is down-regulated by RNA interference but is decreased or eliminated when the cellular level of beta-arrestin2 is diminished. Such reciprocal effects of down-regulated levels of beta-arrestins 1 and 2 are primarily due to differences in the ability of the two forms of beta-arrestins to directly mediate ERK activation. These results are the first to demonstrate reciprocal activity of beta-arrestin isoforms on a signaling pathway and suggest that physiological levels of beta-arrestin1 may act as "dominant-negative" inhibitors of beta-arrestin2-mediated ERK activation.

Regulation of G protein-coupled receptor endocytosis and trafficking by Rab GTPases

G protein-coupled receptors (GPCRs) are integral membrane proteins that, in response to activation by extracellular stimuli, regulate intracellular second messenger levels via their coupling to heterotrimeric G proteins. GPCR activation also initiates a series of molecular events that leads to G protein-coupled receptor kinase-mediated receptor phosphorylation and the binding of beta-arrestin proteins to the intracellular face of the receptor. beta-Arrestin binding not only contributes to the G protein-uncoupling of GPCRs, but also mediates the targeting of many GPCRs for endocytosis in clathrin-coated pits. Several GPCRs internalize as a stable complex with beta-arrestin and the stability of this complex appears to regulate, at least in part, whether the receptors are dephosphorylated in early endosomes and recycled back to the cell surface as fully functional receptors, retained in early endosomes or targeted for degradation in lysosomes. More recently, it has become appreciated that the movement of GPCRs through functionally distinct intracellular membrane compartments is regulated by a variety of Rab GTPases and that the activity of these Rab GTPases may influence GPCR function. Moreover, it appears that GPCRs are not simply passive cargo molecules, but that GPCR activation may directly influence Rab GTPase activity and as such, GPCRs may directly control their own targeting between intracellular compartments. This review provides a synopsis of the current knowledge regarding the role of beta-arrestins and Rab GTPases in regulating the intracellular trafficking and function of GPCRs.

Prevention of ginsenoside-induced desensitization of Ca2+-activated Cl- current by microinjection of inositol hexakisphosphate in Xenopus laevis oocytes: involvement of GRK2 and beta-arrestin I

We demonstrated that ginsenosides, the active ingredient of Panax ginseng, enhance endogenous Ca(2+)-activated Cl(-) currents via Galpha(q/11)-phospholipase C-beta3 pathway in Xenopus laevis oocytes. Moreover, prolonged treatment of ginsenosides induced Cl(-) channel desensitization. However, the molecular mechanisms involved in ginsenoside-induced Cl(-) channel desensitization have not yet been determined precisely. To provide answers to these questions, we investigated the changes in ginsenoside-induced Cl(-) channel desensitization after intraoocyte injection of inositol hexakisphosphate (InsP(6)), which is known to bind beta-arrestins and interfere with beta-arrestin-induced receptor down-regulation, and cRNAs coding beta-arrestin I/II and G-protein receptor kinase 2 (GRK2), which is known to phosphorylate G protein-coupled receptors and attenuate agonist stimulations. When control oocytes were stimulated with ginsenosides, the second, third, and fourth responses to ginsenosides were 69.6 +/- 4.1, 9.2 +/- 2.3, and 2.6 +/- 2.2% of the first responses, respectively. Preintraoocyte injection of InsP(6) before ginsenoside treatment restored ginsenoside effect to initial response levels in a concentration-, time-, and structurally specific manner, in that inositol hexasulfate had no effect. The EC(50) was 13.9 +/- 8.7 microM. Injection of cRNA coding beta-arrestin I but not beta-arrestin II blocked InsP(6) effect on prevention of ginsenoside-induced Cl(-) channel desensitization. Injection of cRNA coding GRK2 abolished ginsenoside effect enhancing Cl(-) current. However, the GRK2-caused loss of ginsenoside effect on Cl(-) current was prevented by coinjection of GRK2 with GRK2-K220R, a dominant-negative mutant of GRK. These results indicate that ginsenoside-induced Cl(-) channel desensitization is mediated via activation of GRK2 and beta-arrestin I.

Enhanced rewarding properties of morphine, but not cocaine, in beta(arrestin)-2 knock-out mice

The reinforcing and psychomotor effects of morphine involve opiate stimulation of the dopaminergic system via activation of mu-opioid receptors (muOR). Both mu-opioid and dopamine receptors are members of the G-protein-coupled receptor (GPCR) family of proteins. GPCRs are known to undergo desensitization involving phosphorylation of the receptor and the subsequent binding of beta(arrestins), which prevents further receptor-G-protein coupling. Mice lacking beta(arrestin)-2 (beta(arr2)) display enhanced sensitivity to morphine in tests of pain perception attributable to impaired desensitization of muOR. However, whether abrogating muOR desensitization affects the reinforcing and psychomotor properties of morphine has remained unexplored. In the present study, we examined this question by assessing the effects of morphine and cocaine on locomotor activity, behavioral sensitization, conditioned place preference, and striatal dopamine release in beta(arr2) knock-out (beta(arr2)-KO) mice and their wild-type (WT) controls. Cocaine treatment resulted in very similar neurochemical and behavioral responses between the genotypes. However, in the beta(arr2)-KO mice, morphine induced more pronounced increases in striatal extracellular dopamine than in WT mice. Moreover, the rewarding properties of morphine in the conditioned place preference test were greater in the beta(arr2)-KO mice when compared with the WT mice. Thus, beta(arr2) appears to play a more important role in the dopaminergic effects mediated by morphine than those induced by cocaine.

The role of phosphorylation in D1 dopamine receptor desensitization: evidence for a novel mechanism of arrestin association

Homologous desensitization of D(1) dopamine receptors is thought to occur through their phosphorylation leading to arrestin association which interdicts G protein coupling. In order to identify the relevant domains of receptor phosphorylation, and to determine how this leads to arrestin association, we created a series of mutated D(1) receptor constructs. In one mutant, all of the serine/threonine residues within the 3rd cytoplasmic domain were altered (3rdTOT). A second construct was created in which only three of these serines (serines 256, 258, and 259) were mutated (3rd234). We also created four truncation mutants of the carboxyl terminus (T347, T369, T394, and T404). All of these constructs were comparable with the wild-type receptor with respect to expression and adenylyl cyclase activation. In contrast, both of the 3rd loop mutants exhibited attenuated agonist-induced receptor phosphorylation that was correlated with an impaired desensitization response. Sequential truncation of the carboxyl terminus of the receptor resulted in a sequential loss of agonist-induced phosphorylation. No phosphorylation was observed with the most severely truncated T347 mutant. Surprisingly, all of the truncated receptors exhibited normal desensitization. The ability of the receptor constructs to promote arrestin association was evaluated using arrestin-green fluorescent protein translocation assays and confocal fluorescence microscopy. The 3rd234 mutant receptor was impaired in its ability to induce arrrestin translocation, whereas the T347 mutant was comparable with wild type. Our data suggest a model in which arrestin directly associates with the activated 3rd cytoplasmic domain in an agonist-dependent fashion; however, under basal conditions, this is sterically prevented by the carboxyl terminus of the receptor. Receptor activation promotes the sequential phosphorylation of residues, first within the carboxyl terminus and then the 3rd cytoplasmic loop, thereby dissociating these domains and allowing arrestin to bind to the activated 3rd loop. Thus, the role of receptor phosphorylation is to allow access of arrestin to its receptor binding domain rather than to create an arrestin binding site per se.

The G protein-coupled receptor GPR4 suppresses ERK activation in a ligand-independent manner

The lysophospholipids, lysophosphatidic acid, sphingosine-1-phosphate, and sphingosylphosphorylcholine (SPC), are bioactive lipid molecules that regulate diverse biological processes. Although the specific G protein-coupled receptors for lysophosphatidic acid and sphingosine-1-phosphate have been well-characterized, much less is known of the SPC receptors. It has been reported that ovarian cancer G protein-coupled receptor 1 (OGR1) is a high affinity receptor for SPC, and its closely related homologue GPR4 is a high affinity receptor for SPC with low affinity for lysophosphatidylcholine (LPC). However, in a functional assay to examine the specificity of ligand binding, we found that neither SPC nor LPC, or other related lysophospholipids, induced internalization of GPR4 from the plasma membrane. In agreement, these lysolipids also did not induce translocation of beta-arrestin2-GFP from the cytosol to the plasma membrane in GPR4 expressing cells. However, when these cells were cotransfected with G protein-coupled receptor kinase 2, in the absence of added ligands, beta-arrestin2-GFP accumulated in cytoplasmic vesicles, reminiscent of vesicular labeling usually observed after agonist stimulation of GPCRs. In addition, neither SPC nor LPC stimulated the binding of GTPgammaS to membranes prepared from GPR4 expressing cells and did not activate ERK1/2. Surprisingly, enforced expression of GPR4 inhibited activation of ERK1/2 induced by several stimuli, including SPC, sphingosine-1-phosphate, and even EGF. Collectively, our results suggest that SPC and LPC are not the ligands for GPR4 and that this receptor may constitutively inhibit ERK1/2 activation.

Regulation of V2 vasopressin receptor degradation by agonist-promoted ubiquitination

The seven-transmembrane-spanning vasopressin V2 receptor (V2R) is a Gs-coupled receptor that is rapidly phosphorylated and internalized following stimulation with the agonist, arginine-vasopressin. Herein, we show that the V2R is ubiquitinated following agonist stimulation. V2R-ubiquitination is not observed in a beta-arrestin1,2 deleted mouse fibroblast cell line and is restored following introduction of beta-arrestin2, thus indicating that beta-arrestin2 is required for the ubiquitination of V2R. A mutant V2R (K268R) that is not ubiquitinated still activates Gs and internalizes with similar kinetics as the wild type receptor. Unstimulated wild type and K268R mutant receptors degrade at similar rates and have comparable half-lives of 217 +/- 17 and 245 +/- 29 min as determined by pulse-chase experiments. However, following agonist stimulation, the rate of receptor degradation for the wild type is enhanced (half-life of 69 +/- 19 min), whereas that of the mutant is only minimally affected (half-life of 188 +/- 11 min). These data suggest that V2R levels are regulated through at least two processes. In the absence of agonist stimulation, a slow degradative pathway operates that is independent of receptor ubiquitination. However, receptor stimulation leads to rapid beta-arrestin2-dependent ubiquitination of the receptor and increased degradation.

The adaptor protein beta-arrestin2 enhances endocytosis of the low density lipoprotein receptor

Endocytosis of the low density lipoprotein (LDL) receptor (LDLR) in coated pits employs the clathrin adaptor protein ARH. Similarly, agonist-dependent endocytosis of heptahelical receptors in coated pits employs the clathrin adaptor beta-arrestin proteins. In mice fed a high fat diet, we found that homozygous deficiency of beta-arrestin2 increased total and LDL plus intermediate-density lipoprotein cholesterol levels by 23 and 53%, respectively (p < 0.05), but had no effect on high density lipoprotein cholesterol levels. We therefore tested whether beta-arrestins could affect the constitutive endocytosis of the LDLR. When overexpressed in cells, beta-arrestin1 and beta-arrestin2 each associated with the LDLR, as judged by co-immunoprecipitation, and augmented LDLR endocytosis by approximately 70%, as judged by uptake of fluorescent LDL. However, physiologic expression levels of only beta-arrestin2, and not beta-arrestin1, enhanced endogenous LDLR endocytosis (by 65%) in stably transfected beta-arrestin1/beta-arrestin2 double-knockout mouse embryonic fibroblasts (MEFs). Concordantly, when RNA interference was used to suppress expression of beta-arrestin2, but not beta-arrestin1, LDLR endocytosis was reduced. Moreover, beta-arrestin2-/- MEFs demonstrated LDLR endocytosis that was 50% less than cognate wild type MEFs. In fusion protein pull-down assays, beta-arrestin2 bound to the LDLR cytoplasmic tail stoichiometrically, and binding was abolished by mutation of LDLR Tyr807 to Ala. Mutation of LDLR cytoplasmic tail Ser833 to Asp enhanced both the affinity of LDLR fusion protein binding to beta-arrestin2, and the efficiency of LDLR endocytosis in cells expressing beta-arrestin2 physiologically. We conclude that beta-arrestin2 can bind to and enhance endocytosis of the LDLR, both in vitro and in vivo, and may thereby influence lipoprotein metabolism.

Multifaceted roles of beta-arrestins in the regulation of seven-membrane-spanning receptor trafficking and signalling

Beta-arrestins are cytosolic proteins that bind to activated and phosphorylated G-protein-coupled receptors [7MSRs (seven-membrane-spanning receptors)] and uncouple them from G-protein-mediated second messenger signalling pathways. The binding of beta-arrestins to 7MSRs also leads to new signals via activation of MAPKs (mitogen-activated protein kinases) such as JNK3 (c-Jun N-terminal kinase 3), ERK1/2 (extracellular-signal-regulated kinase 1/2) and p38 MAPKs. By binding to endocytic proteins [clathrin, AP2 (adapter protein 2), NSF (N -ethylmaleimide-sensitive fusion protein) and ARF6 (ADP-ribosylation factor 6)], beta-arrestins also serve as adapters to link the receptors to the cellular trafficking machinery. Agonist-promoted ubiquitination of beta-arrestins is a prerequisite for their role in receptor internalization, as well as a determinant of the differing trafficking patterns of distinct classes of receptors. Recently, beta-arrestins have also been implicated as playing novel roles in cellular chemotaxis and apoptosis. By virtue of their ability to bind, in a stimulus-dependent fashion, to 7MSRs as well as to different classes of cellular proteins, beta-arrestins serve as versatile adapter proteins that regulate the signalling and trafficking of the receptors.

Agonist- and protein kinase C-induced phosphorylation have similar functional consequences for gastrin-releasing peptide receptor signaling via Gq

Acute desensitization of many guanine nucleotide-binding protein-coupled receptors (GPCRs) requires receptor phosphorylation and subsequent binding of an arrestin. GPCRs are substrates for phosphorylation by several classes of kinases. Gastrin-releasing peptide receptor (GRPr) is phosphorylated by a kinase other than protein kinase C (PKC) after exposure to agonist and is also a substrate for PKC-dependent phosphorylation after treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA). Using GRPr mutants, we examined receptor domains required for agonist- and TPA-induced phosphorylation of GRPr and consequences of these phosphorylation events on GRPr signaling via Gq. Agonist- and TPA-stimulated GRPr phosphorylation in cells require an intact carboxyl terminal domain (CTD). GRPr is phosphorylated in vitro by GPCR kinase 2 (GRK2) and multiple PKC isoforms. An intact DRY motif is required for agonist-stimulated phosphorylation in cells, and agonist-dependent GRK2 phosphorylation in vitro. Although GRPr CTD mutants do not show enhanced in vitro coupling to Gq relative to intact GRPr, CTD mutants have more potent Gq-dependent signaling in cells. Acute desensitization involves CTD-independent processes because desensitization can precede ligand binding in intact GRPr and CTD mutants. TPA-mediated impairment of GRPr-Gq signaling in cells also requires an intact CTD. Similar to GRK2 phosphorylation, PKC phosphorylation reduces GRPr-Gq coupling by approximately 80% in vitro. Arrestin translocation to plasma membrane requires agonist, an intact DRY motif, and GRPr phosphorylation. Therefore, agonist- and PKC-induced GRPr phosphorylation sites are in nearby regions of the receptor, and phosphorylation at both sites has similar functional consequences for Gq signaling.

Heterodimerization of substance P and mu-opioid receptors regulates receptor trafficking and resensitization

The micro-opioid receptor (MOR1) and the substance P receptor (NK1) coexist and functionally interact in nociceptive brain regions; however, a molecular basis for this interaction has not been established. Using coimmunoprecipitation and bioluminescence resonance energy transfer (BRET), we show that MOR1 and NK1 can form heterodimers in HEK 293 cells coexpressing the two receptors. Although NK1-MOR1 heterodimerization did not substantially change the ligand binding and signaling properties of these receptors, it dramatically altered their internalization and resensitization profile. Exposure of the NK1-MOR1 heterodimer to the MOR1-selective ligand [D-Ala2,Me-Phe4,Gly5-ol]enkephalin (DAMGO) promoted cross-phosphorylation and cointernalization of the NK1 receptor. Conversely, exposure of the NK1-MOR1 heterodimer to the NK1-selective ligand substance P (SP) promoted cross-phosphorylation and cointernalization of the MOR1 receptor. In cells expressing MOR1 alone, beta-arrestin directs the receptors to clathrin-coated pits, but does not internalize with the receptor. In cells expressing NK1 alone, beta-arrestin internalizes with the receptor into endosomes. Interestingly, in cells coexpressing MOR1 and NK1 both DAMGO and SP induced the recruitment of beta-arrestin to the plasma membrane and cointernalization of NK1-MOR1 heterodimers with beta-arrestin into the same endosomal compartment. Consequently, resensitization of MOR1-dependent receptor functions was severely delayed in coexpressing cells as compared with cells expressing MOR1 alone. Together, our findings indicate that MOR1 by virtue of its physical interaction with NK1 is sequestered via an endocytotic pathway with delayed recycling and resensitization kinetics.

Mapping the arrestin-receptor interface. Structural elements responsible for receptor specificity of arrestin proteins

Arrestins selectively bind to phosphorylated activated forms of their cognate G protein-coupled receptors. Arrestin binding prevents further G protein activation and often redirects signaling to other pathways. The comparison of the high-resolution crystal structures of arrestin2, visual arrestin, and rhodopsin as well as earlier mutagenesis and peptide inhibition data collectively suggest that the elements on the concave sides of both arrestin domains most likely participate in receptor binding directly, thereby dictating its receptor preference. Using comparative binding of visual arrestin/arrestin2 chimeras to the preferred target of visual arrestin, light-activated phosphorylated rhodopsin (PRh*), and to the arrestin2 target, phosphorylated activated m2 muscarinic receptor (P-m2 mAChR*), we identified the elements that determine the receptor specificity of arrestins. We found that residues 49-90 (beta-strands V and VI and adjacent loops in the N-domain) and 237-268 (beta-strands XV and XVI in the C-domain) in visual arrestin and homologous regions in arrestin2 are largely responsible for their receptor preference. Only 35 amino acids (22 of which are nonconservative substitutions) in the two elements are different. Simultaneous exchange of both elements between visual arrestin and arrestin2 fully reverses their receptor specificity, demonstrating that these two elements in the two domains of arrestin are necessary and sufficient to determine their preferred receptor targets.

Functional role of the NPxxY motif in internalization of the type 2 vasopressin receptor in LLC-PK1 cells

Interaction of the type 2 vasopressin receptor (V2R) with hormone causes desensitization and internalization. To study the role of the V2R NPxxY motif (which is involved in the clathrin-mediated endocytosis of several other receptors) in this process, we expressed FLAG-tagged wild-type V2R and a Y325F mutant V2R in LLC-PK1a epithelial cells that have low levels of endogenous V2R. Both proteins had a similar apical (35%) and basolateral (65%) membrane distribution. Substitution of Tyr325 with Phe325 prevented ligand-induced internalization of V2R determined by [3H]AVP binding and immunofluorescence but did not prevent ligand binding or signal transduction via adenylyl cyclase. Desensitization and resensitization of the V2R-Y325F mutation occurred independently of internalization. The involvement of clathrin in V2R downregulation was also shown by immunogold electron microscopy. We conclude that the NPxxY motif of the V2R is critically involved in receptor downregulation via clathrin-mediated internalization. However, this motif is not essential for the apical/basolateral sorting and polarized distribution of the V2R in LLC-PK1a cells or for adenylyl cyclase-mediated signal transduction.

The third intracellular loop and carboxyl tail of neurokinin 1 and 3 receptors determine interactions with beta-arrestins

Tachykinins interact with three neurokinin receptors (NKRs) that are often coexpressed by the same cell. Cellular responses to tachykinins depend on the NKR subtype that is activated. We compared the colocalization of NK1R and NK3R with beta-arrestins 1 and 2, which play major roles in receptor desensitization, endocytosis, and signaling. In cells expressing NK1R, the selective agonist Sar-Met-substance P induced rapid translocation of beta-arrestins 1 and 2 from the cytosol to the plasma membrane and then endosomes, indicative of interaction with both isoforms. In contrast, the NK3R interacted transiently with only beta-arrestin 2 at the plasma membrane. Despite these differences, both NK1R and NK3R similarly desensitized, internalized, and activated MAP kinases. Because interactions with beta-arrestins can explain differences in the rate of receptor resensitization, we compared resensitization of agonist-induced Ca2+ mobilization. The NK1R resensitized greater than twofold more slowly than the NK3R. Replacement of intracellular loop 3 and the COOH tail of the NK1R with comparable domains of the NK3R diminished colocalization of the NK1R with beta-arrestin 1 and accelerated resensitization to that of the NK3R. Thus loop 3 and the COOH tail specify colocalization of the NK1R with beta-arrestin 1 and determine the rate of resensitization.

Dual inhibition of beta-adrenergic and angiotensin II receptors by a single antagonist: a functional role for receptor-receptor interaction in vivo

BACKGROUND

Although the renin-angiotensin and the beta-adrenergic systems are interrelated, a direct interaction between beta-adrenergic receptors (betaARs) and angiotensin II type 1 receptors (AT1Rs) has not been identified.

METHODS AND RESULTS

Here, we provide evidence for a functional and physiological interaction between 2 G protein-coupled receptors: the betaAR and the AT1R. Selective blockade of betaARs in mouse cardiomyocytes inhibits angiotensin-induced contractility with an IC50 that is similar to its inhibition of isoproterenol-mediated contractility. Furthermore, administration of the angiotensin receptor blocker valsartan to intact mice results in a significant reduction in the maximal response to catecholamine-induced elevation of heart rate. The mechanism for this transinhibitory effect of beta-blockers and angiotensin receptor blockers is through receptor-G protein uncoupling; ie, beta-blockers interfere with AT1R-Gq coupling, and valsartan interferes with betaAR-Gs coupling. Finally, we demonstrate that AT1Rs and betaARs form constitutive complexes that are not affected by ligand stimulation. As a result of these interactions, a single receptor antagonist effectively blocks downstream signaling and trafficking of both receptors simultaneously.

CONCLUSIONS

We show that direct interactions between betaARs and AT1Rs may have profound consequences on the overall response to drugs that antagonize these receptors.

Mu-opioid receptor desensitization: role of receptor phosphorylation, internalization, and representation

It is generally accepted that the internalization and desensitization of mu-opioid receptor (MOR) involves receptor phosphorylation and beta-arrestin recruitment. However, a mutant MOR, which is truncated after the amino acid residue Ser363 (MOR363D), was found to undergo phosphorylation-independent internalization and desensitization. As expected, MOR363D, missing the putative agonist-induced phosphorylation sites, did not exhibit detectable agonist-induced phosphorylation. MOR363D underwent slower internalization as reflected in the attenuation of membrane translocation of beta-arrestin 2 when compared with wild type MOR, but the level of receptor being internalized was similar to that of wild type MOR after 4 h of etorphine treatment. Furthermore, MOR363D was observed to desensitize faster than that of wild type MOR upon agonist activation. Surface biotinylation assay demonstrated that the wild type receptors recycled back to membrane after agonist-induced internalization, which contributed to the receptor resensitization and thus partially reversed the receptor desensitization. On the contrary, MOR363D did not recycle after internalization. Hence, MOR desensitization is controlled by the receptor internalization and the recycling of internalized receptor to cell surface in an active state. Taken together, our data indicated that receptor phosphorylation is not absolutely required in the internalization, but receptor phosphorylation and subsequent beta-arrestin recruitment play important roles in the resensitization of internalized receptors.

Independent beta-arrestin 2 and G protein-mediated pathways for angiotensin II activation of extracellular signal-regulated kinases 1 and 2

Stimulation of a mutant angiotensin type 1A receptor (DRY/AAY) with angiotensin II (Ang II) or of a wild-type receptor with an Ang II analog ([sarcosine1,Ile4,Ile8]Ang II) fails to activate classical heterotrimeric G protein signaling but does lead to recruitment of beta-arrestin 2-GFP and activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) (maximum stimulation approximately 50% of wild type). This G protein-independent activation of mitogen-activated protein kinase is abolished by depletion of cellular beta-arrestin 2 but is unaffected by the PKC inhibitor Ro-31-8425. In parallel, stimulation of the wild-type angiotensin type 1A receptor with Ang II robustly stimulates ERK1/2 activation with approximately 60% of the response blocked by the PKC inhibitor (G protein dependent) and the rest of the response blocked by depletion of cellular beta-arrestin 2 by small interfering RNA (beta-arrestin dependent). These findings imply the existence of independent G protein- and beta-arrestin 2-mediated pathways leading to ERK1/2 activation and the existence of distinct "active" conformations of a seven-membrane-spanning receptor coupled to each.

Real-time imaging of green fluorescent protein-tagged beta 2-adrenergic receptor distribution in living cells

In an attempt to investigate the subcellular trafficking of beta(2)-adrenergic receptor (beta(2)AR) in living cells, we performed real-time imaging of beta(2)AR tagged with green fluorescent protein (GFP). We transiently transfected a chimera construct of beta(2)AR and GFP (beta(2)AR-GFP) into HEK 293 cells, primary cultured rat hippocampal neurons and cortical neuronal cells, and then compared the dynamic changes in subcellular localization of beta(2)AR-GFP in these live cells. In the absence of ligands, beta(2)AR-GFP fluorescence was detected predominantly on the plasma membrane in HEK 293 cells as well as on the surface of cell somata and dendrites in cortical neuronal cells. In contrast, in hippocampal neurons, beta(2)AR-GFP was diffusely distributed not only on the surface of cells but in the whole cell somata and dendrites. In HEK 293 cells, cortical neuronal cells and cortical glial cells, time-lapse images showed the rapid appearance of a punctate distribution pattern that became more numerous over the 15-min course of agonist exposure. Semiquantitative analysis revealed the time-course internalization of beta(2)AR-GFP in a single living cell. In hippocampal neurons, beta(2)AR-GFP distribution became scattered both in cell somata and dendrites following agonist exposure. Three-dimensional analysis of time-lapse images revealed a significant portion of beta(2)AR-GFP was distributed in endosomal compartments, along with Alexa 546-labeled transferrin, in all types of cells. Our results demonstrate spatial and temporal redistribution pattern of beta(2)AR in living non-neuronal cells and neuronal cells.

High-content assays for ligand regulation of G-protein-coupled receptors

High-content assays rely on the imaging of cellular events. They can be used to monitor the activation of G-protein-coupled receptors (or other receptors), their internalization into the cell, or alterations in their amount. In addition, multiplexed assays can provide further information about the characteristics of the receptor. Recent improvements in throughput using high-content screening platforms means that such assays are now an integral element of functional analysis in the drug discovery process.

The association of arrestin-3 with the follitropin receptor depends on receptor activation and phosphorylation

We have recently shown that the binding of arrestin-3 to the lutropin receptor (LHR) is dependent mostly on receptor activation rather than on phosphorylation. The experiments presented here were designed to test the involvement of these two events in the association of arrestin-3 with the closely related follitropin receptor (FSHR). Activation of the FSHR leads to the phosphorylation of residues in the first and third intracellular loops. Mutation of the phosphorylation sites in the third intracellular loop of the rat (r) FSHR partially reduces phosphorylation but has no effect on arrestin-3 association. Mutation of the phosphorylation sites in the first intracellular loop abolishes phosphorylation and arrestin-3 association. Dominant-negative mutants of G protein-coupled receptor kinase (GRKs) 2 and 6 inhibit rFSHR phosphorylation to the same extent but only the dominant-negative mutant of GRK2 inhibits arrestin-3 association. Two mutations of the rFSHR (D389N and Y530F) that impair activation and abolish phosphorylation also impair arrestin-3 binding. GRK2 restores the phosphorylation of both mutants but it restores arrestin-3 association only to the D389N mutant. We conclude that, in contrast to the data obtained with the LHR, the association of arrestin-3 with the FSHR is dependent on receptor phosphorylation. The phosphorylation of the third intracellular loop residues is not needed for arrestin-3 association, however.

G-protein-coupled receptor (GPCR) kinase phosphorylation and beta-arrestin recruitment regulate the constitutive signaling activity of the human cytomegalovirus US28 GPCR

Phosphorylation of G-protein-coupled receptors (GPCRs) by GRKs and subsequent recruitment of beta-arrestins to agonist-occupied receptors serves to terminate or attenuate signaling by blocking G-proteins from further interaction with the receptors. Human cytomegalovirus encodes a GPCR termed US28 that is homologous to the human chemokine family of GPCRs but differs from the cellular receptors in that it maintains high constitutive activity in the absence of agonist. Although US28 is constitutively active, mechanisms that regulate this activity are unknown. We provide evidence that US28 is constitutively phosphorylated by GRKs in cells and that in consequence, beta-arrestin 2 is localized to the plasma membrane. Deletion of the carboxyl terminal 40 amino acids in US28 generates a receptor that is severely impaired in its ability to become phosphorylated and recruit beta-arrestin and accordingly demonstrates increased inositol phosphate signaling. This result indicates that the carboxyl terminus of US28 contains an important signaling regulatory region and mutational analysis deleting carboxyl terminal serines identified serine 323 as a critical residue within this region. In addition, overexpression of wild type GRK5 leads to hyperphosphorylation of US28 that results in a decrease of inositol phosphate accumulation. These results are consistent with the hypothesis that GRK phosphorylation and recruitment of beta-arrestin to the US28 viral GPCR attenuates signaling to the traditional Galphaq-stimulated inositol phosphate pathway. Finally, in contrast to the results with inositol phosphate signaling, we provide evidence that the US28 carboxyl-terminal phosphorylation sites and beta-arrestin-interacting domain are required for maximal activation of the p38 mitogen-activated protein kinase. Taken together, these results indicate that US28 interacts with these important regulatory proteins to control multiple aspects of signal transmission. Understanding the regulation of viral GPCRs by GRKs and beta-arrestins will provide important new insights into not only aspects of viral pathogenesis but also basic mechanisms of receptor signaling.

G Protein-Coupled Receptor Desensitization as a Measure of Signaling: Modeling of Arrestin Recruitment to Activated CCK-B Receptors

Gastrin is one of the principle hormonal mediators of gastric acid secretion, and its cognate receptor (CCK-B) is a member of the superfamily of GPCRs. Patients with hypergastrinemia may present with a variety of symptoms, including gastric ulcers or malignant tumors. Thus, the molecular mechanisms that terminate CCK-B receptor signaling, as well as an ability to measure gastrin bioactivity in a timely manner, have important clinical implications. In order to assess CCK-B receptor regulation, we have constructed a single cell biosensor containing the CCK-B receptor and an arrestin/GFP chimera. The gastrin biosensor responded to both immunologically detectable gastrin-17 and undetectable pentagastrin, and was able to determine the gastrin bioactivity of serum from a patient with clinical hypergastrinemia. We determined that the CCK-B receptor binds arrestin with a pharmacology mirroring CCK-B receptor signaling through inositol phosphate, and that the rate of arrestin dissociation from internalized receptor mirrors receptor recycling to the plasma membrane. Moreover, the CCK-B recycling rate is intermediate between that of Class A GPCRs such as the beta2-adrenergic receptor and Class B GPCRs such as the vasopressin type 2 receptor. Mathematical modeling of these results indicates that a common receptor conformation may underlie both CCK-B signaling and desensitization. In addition to its use in drug screening, this methodology should generalize to other receptors for use in diagnosis and monitoring of bioactive ligands involved in GPCR-based disease.

Differential regulation of beta-arrestin 1 and beta-arrestin 2 gene expression in rat brain by morphine

Beta-arrestins are a family of regulatory and scaffold proteins functioning in signal transduction of G protein-coupled receptors including opioid receptors. Upon agonist stimulation, beta-arrestins bind to opioid receptors phosphorylated by G protein-coupled receptor kinases and promote receptor internalization and desensitization. Studies indicated that beta-arrestins are required in the development of morphine tolerance in mice. In the current study, we investigated the potential regulatory effects of morphine administration on beta-arrestin 1 and beta-arrestin 2 mRNA levels in different brain regions in rat using in situ hybridization method. Our results showed that the acute morphine administration (10 mg/kg) resulted in approximately 30% reduction in both beta-arrestin 1 and beta-arrestin 2 mRNA levels in hippocampus while the chronic morphine treatment (10 mg/kg, b.i.d., for 9 days) caused no significant change in level of either beta-arrestin mRNA. In locus coeruleus, both acute and chronic morphine treatments resulted in significant decreases (over 50%) in beta-arrestin 1 mRNA level but failed to induce any change in the level of beta-arrestin 2 gene expression. The acute morphine administration had no significant effect on beta-arrestin 1 or beta-arrestin 2 mRNA level in periaqueductal gray and cerebral cortex. However, after chronic morphine treatment, beta-arrestin 2 mRNA level decreased by 40% in periaqueductal gray and increased by 25% in cerebral cortex, in strong contrast to the unchanged beta-arrestin 1 mRNA level in these two brain regions. Furthermore, spontaneous or naloxone-precipitated withdrawal of morphine that did not affect the level of beta-arrestin 1 mRNA resulted in an aberrant increase (100% over control) in beta-arrestin 2 mRNA level in hippocampus. Our results thus demonstrated for the first time that opiate administration regulates level of beta-arrestin mRNAs in brain and the expression of beta-arrestin 1 and beta-arrestin 2 subtypes is differentially regulated in locus coeruleus, periaqueductal gray, and cerebral cortex by morphine. These data suggest that beta-arrestin 1 and beta-arrestin 2 may play different roles in the development of opioid tolerance and dependence.

Trafficking patterns of beta-arrestin and G protein-coupled receptors determined by the kinetics of beta-arrestin deubiquitination

Agonist-dependent internalization of G protein-coupled receptors via clathrin-coated pits is dependent on the adaptor protein beta-arrestin, which interacts with elements of the endocytic machinery such as AP2 and clathrin. For the beta(2)-adrenergic receptor (beta(2)AR) this requires ubiquitination of beta-arrestin by E3 ubiquitin ligase, Mdm2. Based on trafficking patterns and affinity of beta-arrestin, G protein-coupled receptors are categorized into two classes. For class A receptors (e.g. beta(2)AR), which recycle rapidly, beta-arrestin directs the receptors to clathrin-coated pits but does not internalize with them. For class B receptors (e.g. V2 vasopressin receptors), which recycle slowly, beta-arrestin internalizes with the receptor into endosomes. In COS-7 and human embryonic kidney (HEK)-293 cells, stimulation of the beta(2)AR or V2 vasopressin receptor leads, respectively, to transient or stable beta-arrestin ubiquitination. The time course of ubiquitination and deubiquitination of beta-arrestin correlates with its association with and dissociation from each type of receptor. Chimeric receptors, constructed by switching the cytoplasmic tails of the two classes of receptors (beta(2)AR and V2 vasopressin receptors), demonstrate reversal of the patterns of both beta-arrestin trafficking and beta-arrestin ubiquitination. To explore the functional consequences of beta-arrestin ubiquitination we constructed a yellow fluorescent protein-tagged beta-arrestin2-ubiquitin chimera that cannot be deubiquitinated by cellular deubiquitinases. This "permanently ubiquitinated" beta-arrestin did not dissociate from the beta(2)AR but rather internalized with it into endosomes, thus transforming this class A receptor into a class B receptor with respect to its trafficking pattern. Overexpression of this beta-arrestin ubiquitin chimera in HEK-293 cells also results in enhancement of beta(2)AR internalization and degradation. In the presence of N-ethylmaleimide (an inhibitor of deubiquitinating enzymes), coimmunoprecipitation of the receptor and beta-arrestin was increased dramatically, suggesting that deubiquitination of beta-arrestin triggers its dissociation from the receptor. Thus the ubiquitination status of beta-arrestin determines the stability of the receptor-beta-arrestin complex as well as the trafficking pattern of beta-arrestin.

The nature of the arrestin x receptor complex determines the ultimate fate of the internalized receptor

The vast majority of G protein-coupled receptors are desensitized by a uniform two-step mechanism: phosphorylation of an active receptor followed by arrestin binding. The arrestin x receptor complex is then internalized. Internalized receptor can be recycled back to the plasma membrane (resensitization) or targeted to lysosomes for degradation (down-regulation). The intracellular compartment where this choice is made and the molecular mechanisms involved are largely unknown. Here we used two arrestin2 mutants that bind with high affinity to phosphorylated and unphosphorylated agonist-activated beta 2-adrenergic receptor to manipulate the receptor-arrestin interface. We found that mutants support rapid internalization of beta 2-adrenergic receptor similar to wild type arrestin2. At the same time, phosphorylation-independent arrestin2 mutants facilitate receptor recycling and sharply reduce the rate of receptor loss, effectively protecting beta 2-adrenergic receptor from down-regulation even after very long (up to 24 h) agonist exposure. Phosphorylation-independent arrestin2 mutants dramatically reduce receptor phosphorylation in response to an agonist both in vitro and in cells. Interestingly, co-expression of high levels of beta-adrenergic receptor kinase restores receptor down-regulation in the presence of mutants to the levels observed with wild type arrestin2. Our data suggest that unphosphorylated receptor internalized in complex with mutant arrestins recycles faster than phosphoreceptor and is less likely to get degraded. Thus, targeted manipulation of the characteristics of an arrestin protein that binds to a G protein-coupled receptors can dramatically change receptor trafficking and its ultimate fate in a cell.

Subcellular localization of beta-arrestins is determined by their intact N domain and the nuclear export signal at the C terminus

beta-Arrestin1 and beta-arrestin2 play a key role in the regulation of G protein-coupled receptor-mediated signaling, whereas the subcellular distribution of beta-arrestin1 and beta-arrestin2 has been shown to be quite different. In this study, we found that although both beta-arrestin1 and beta-arrestin2 are able to interact with ubiquitin-protein isopeptide ligase (E3) Mdm2, only expression of beta-arrestin2 leads to the relocalization of Mdm2 from the nucleus to the cytoplasm. Further study reveals that beta-arrestin2 but not beta-arrestin1 shuttles between the cytoplasm and nucleus in a leptomycin B-sensitive manner. A hydrophobic amino acid-rich region (VXXXFXXLXL) at the C terminus of beta-arrestin2 was further demonstrated to serve as a nuclear export signal responsible for the extranuclear localization of beta-arrestin2. In the corresponding region of beta-arrestin1, there is a single amino acid difference (Glu instead of Leu in beta-arrestin2), and mutation of Glu to Leu conferred to beta-arrestin1 similar subcellular distribution to that of beta-arrestin2. Moreover, data from a series of deletion mutations demonstrated that the N domain (residues 1-185) was indispensable for the nuclear localization of both beta-arrestins, and the results from a Val to Asp point mutation in the N domain also supported this notion. In addition, our data showed that nucleocytoplasmic shuttling of beta-arrestin2 was required, via protein/protein interaction, for the cytoplasmic relocalization of Mdm2 and JNK3, another well known beta-arrestin2-binding protein. Our study thus suggests that both the nuclear export signal motif and the N domain of beta-arrestins are critical for the regulation of their subcellular localization and that beta-arrestin2 may modulate the function of its binding partners such as Mdm2 and JNK3 by alteration of their subcellular distribution.

Role of the cytoplasmic tails of CXCR1 and CXCR2 in mediating leukocyte migration, activation, and regulation

IL-8 (or CXCL8) activates the receptors CXCR1 (IL-8RA) and CXCR2 (IL-8RB) to induce chemotaxis in leukocytes, but only CXCR1 mediates cytotoxic and cross-regulatory signals. This may be due to the rapid internalization of CXCR2. To investigate the roles of the intracellular domains in receptor regulation, wild-type, chimeric, phosphorylation-deficient, and cytoplasmic tail (C-tail) deletion mutants of both receptors were expressed in RBL-2H3 cells and studied for cellular activation, receptor phosphorylation, desensitization, and internalization. All but one chimeric receptor bound IL-8 and mediated signal transduction, chemotaxis, and exocytosis. Upon IL-8 activation, the chimeric receptors underwent receptor phosphorylation and desensitization. One was resistant to internalization, yet it mediated normal levels of beta-arrestin 2 (beta arr-2) translocation. The lack of internalization by this receptor may be due to its reduced association with beta arr-2 and the adaptor protein-2 beta. The C-tail-deleted and phosphorylation-deficient receptors were resistant to receptor phosphorylation, desensitization, arrestin translocation, and internalization. They also mediated greater phosphoinositide hydrolysis and exocytosis and sustained Ca(2+) mobilization, but diminished chemotaxis. These data indicate that phosphorylation of the C-tails of CXCR1 and CXCR2 are required for arrestin translocation and internalization, but are not sufficient to explain the rapid internalization of CXCR2 relative to CXCR1. The data also show that receptor internalization is not required for chemotaxis. The lack of receptor phosphorylation was correlated with greater signal transduction but diminished chemotaxis, indicating that second messenger production, not receptor internalization, negatively regulates chemotaxis.

The stability of the G protein-coupled receptor-beta-arrestin interaction determines the mechanism and functional consequence of ERK activation

By binding to agonist-activated G protein-coupled receptors (GPCRs), beta-arrestins mediate homologous receptor desensitization and endocytosis via clathrin-coated pits. Recent data suggest that beta-arrestins also contribute to GPCR signaling by acting as scaffolds for components of the ERK mitogen-activated protein kinase cascade. Because of these dual functions, we hypothesized that the stability of the receptor-beta-arrestin interaction might affect the mechanism and functional consequences of GPCR-stimulated ERK activation. In transfected COS-7 cells, we found that angiotensin AT1a and vasopressin V2 receptors, which form stable receptor-beta-arrestin complexes, activated a beta-arrestin-bound pool of ERK2 more efficiently than alpha 1b and beta2 adrenergic receptors, which form transient receptor-beta-arrestin complexes. We next studied chimeric receptors in which the pattern of beta-arrestin binding was reversed by exchanging the C-terminal tails of the beta2 and V2 receptors. The ability of the V2 beta 2 and beta 2V2 chimeras to activate beta-arrestin-bound ERK2 corresponded to the pattern of beta-arrestin binding, suggesting that the stability of the receptor-beta-arrestin complex determined the mechanism of ERK2 activation. Analysis of covalently cross-linked detergent lysates and cellular fractionation revealed that wild type V2 receptors generated a larger pool of cytosolic phospho-ERK1/2 and less nuclear phospho-ERK1/2 than the chimeric V2 beta 2 receptor, consistent with the cytosolic retention of beta-arrestin-bound ERK. In stably transfected HEK-293 cells, the V2 beta 2 receptor increased ERK1/2-mediated, Elk-1-driven transcription of a luciferase reporter to a greater extent than the wild type V2 receptor. Furthermore, the V2 beta 2, but not the V2 receptor, was capable of eliciting a mitogenic response. These data suggest that the C-terminal tail of a GPCR, by determining the stability of the receptor-beta-arrestin complex, controls the extent of beta-arrestin-bound ERK activation, and influences both the subcellular localization of activated ERK and the physiologic consequences of ERK activation.

Desensitization, internalization, and signaling functions of beta-arrestins demonstrated by RNA interference

Beta-arrestins bind to activated G protein-coupled receptor kinase-phosphorylated receptors, which leads to their desensitization with respect to G proteins, internalization via clathrin-coated pits, and signaling via a growing list of "scaffolded" pathways. To facilitate the discovery of novel adaptor and signaling roles of beta-arrestins, we have developed and validated a generally applicable interfering RNA approach for selectively suppressing beta-arrestins 1 or 2 expression by up to 95%. Beta-arrestin depletion in HEK293 cells leads to enhanced cAMP generation in response to beta(2)-adrenergic receptor stimulation, markedly reduced beta(2)-adrenergic receptor and angiotensin II receptor internalization and impaired activation of the MAP kinases ERK 1 and 2 by angiotensin II. This approach should allow discovery of novel signaling and regulatory roles for the beta-arrestins in many seven-membrane-spanning receptor systems.

Phosphorylation of key serine residues is required for internalization of the complement 5a (C5a) anaphylatoxin receptor via a beta-arrestin, dynamin, and clathrin-dependent pathway

The human complement 5a (C5a) anaphylatoxin receptor (CD88) is a G protein-coupled receptor involved in innate host defense and inflammation. Upon agonist binding, C5a receptor (C5aR) undergoes rapid phosphorylation on the six serine residues present in the C-terminal region followed by desensitization and internalization. Using confocal immunofluorescence microscopy and green fluorescent protein-tagged beta-arrestins (beta-arr 1- and beta-arr 2-EGFP) we show a persistent complex between C5aR and beta-arrestins to endosomal compartments. Serine residues in the C5aR C terminus were identified that control the intracellular trafficking of the C5aR-arrestin complex in response to C5a. Two phosphorylation mutants C5aR-A(314,317,327,332) and C5aR-A(314,317,332,334), which are phosphorylated only on Ser(334)/Ser(338) and Ser(327)/Ser(338), respectively, recruited beta-arr 1 and were internalized. In contrast, the phosphorylation-deficient receptors C5aR-A(334,338) and C5aR-A(332,334,338) were not internalized even though observations by confocal microscopy indicated that beta-arr 1-EGFP and/or beta-arr 2-EGFP could be recruited to the plasma membrane. Altogether the results indicate that C5aR activation is able to promote a loose association with beta-arrestins, but phosphorylation of either Ser(334)/Ser(338) or Ser(327)/Ser(338) is necessary and sufficient for the formation of a persistent complex. In addition, it was observed that C5aR endocytosis was inhibited by the expression of the dominant negative mutants of dynamin (K44E) and beta-arrestin 1 (beta-arr 1-(319-418)-EGFP). Thus, the results suggest that the C5aR is internalized via a pathway dependent on beta-arrestin, clathrin, and dynamin.

Human substance P receptor lacking the C-terminal domain remains competent to desensitize and internalize

Substance P receptor (SPR) and its naturally occurring splice-variant, lacking the C-terminal tail, are found in brain and spinal cord. Whether C-terminally truncated SPR desensitizes like full-length SPR is controversial. We used a multivaried approach to determine whether human SPR (hSPR) and a C-terminally truncated mutant, hSPRDelta325, differ in their desensitization and internalization. In HEK-293 cells expressing either hSPRDelta325 or hSPR, SP-induced desensitization of the two receptors was similar when measured by inositol triphosphate accumulation or by transient translocation of coexpressed PKCbetaII-GFP to the plasma membrane. Moreover, translocation of beta-arrestin 1 or 2-GFP (betaarr1-GFP or betaarr2-GFP) to the plasma membrane, and receptor internalization were also similar. However, hSPR and hSPRDelta325 differ in their phosphorylation and in their ability to form beta-arrestin-containing endocytic vesicles. Unlike hSPR, hSPRDelta325 is not phosphorylated to a detectable level in intact HEK293 cells, and whereas hSPR forms vesicles containing either betaarr1-GFP or betaarr2-GFP, hSPRDelta325 does not form any vesicles with betaarr1-GFP, and forms fewer vesicles with betaarr2-GFP. We conclude that truncated hSPR undergoes agonist-dependent desensitization and internalization without detectable receptor phosphorylation.

Neurokinin-1 versus mu-opioid receptor binding in rat nucleus tractus solitarius after single and recurrent intermittent hypoxia

G protein-coupled excitatory neurokinin-1 and inhibitory mu-opioid receptors exist in respiratory brainstem with their peptides and influence breathing. To assess their putative role in respiratory responses to hypoxia, neurokinin-1, and mu-opioid receptor binding was determined in the respiratory nucleus tractus solitarius of the mature rat after single and recurrent intermittent hypoxia versus normoxia. Hypoxia comprised six 5-min bouts of 8% O(2)-92% N(2) interceded by 5-min bouts in 21% O(2)-79% N(2) (normoxia), either on 6 consecutive days (recurrent intermittent hypoxia) or on the 6th day only (single intermittent hypoxia). Controls comprised six daily sessions in normoxia. To examine the plasticity in receptor response, brains were collected 5min, 2h, or 24h after the last gaseous exposure. Sections from each brainstem underwent quantitative film autoradiography with iodinated substance P and DAMGO for neurokinin-1 and mu-opioid receptors, respectively. Neurokinin-1 receptor binding decreased 5min after single and recurrent hypoxia and 2h after recurrent hypoxia, whereas mu-opioid binding remained unchanged. The binding of both receptors increased 24h after recurrent intermittent hypoxia. Neurokinin versus mu-opioid binding differences immediately posthypoxia might affect physiological responses to episodic hypoxia.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

Homo- and hetero-oligomerization of thyrotropin-releasing hormone (TRH) receptor subtypes. Differential regulation of beta-arrestins 1 and 2

G-protein-coupled receptors (GPCRs) are regulated by a complex network of mechanisms such as oligomerization and internalization. Using the GPCR subtypes for thyrotropin-releasing hormone (TRHR1 and TRHR2), the aim of this study was to determine if subtype-specific differences exist in the trafficking process. If so, we wished to determine the impact of homo- and hetero-oligomerization on TRHR subtype trafficking as a potential mechanism for the differential cellular responses induced by TRH. Expression of either beta-arrestin 1 or 2 promoted TRHR1 internalization. In contrast, only beta-arrestin 2 could enhance TRHR2 internalization. The preference for beta-arrestin 2 by TRHR2 was supported by the impairment of TRHR2 trafficking in mouse embryonic fibroblasts (MEFs) from either a beta-arrestin 2 knockout or a beta-arrestin 1/2 knockout, while TRHR1 trafficking was only abolished in MEFs lacking both beta-arrestins. The differential beta-arrestin-dependence of TRHR2 was directly measured in live cells using bioluminescence resonance energy transfer (BRET). Both BRET and confocal microscopy were also used to demonstrate that TRHR subtypes form hetero-oligomers. In addition, these hetero-oligomers have altered internalization kinetics compared with the homo-oligomer. The formation of TRHR1/2 heteromeric complexes increased the interaction between TRHR2 and beta-arrestin 1. This may be due to conformational differences between TRHR1/2 hetero-oligomers versus TRHR2 homo-oligomers as a mutant TRHR1 (TRHR1 C335Stop) that does not interact with beta-arrestins, could also enhance TRHR2/beta-arrestin 1 interaction. This study demonstrates that TRHR subtypes are differentially regulated by the beta-arrestins and also provides the first evidence that the interactions of TRHRs with beta-arrestin may be altered by hetero-oligomer formation.

The Cellular Distribution of Fluorescently Labeled Arrestins Provides a Robust, Sensitive, and Universal Assay for Screening G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) have proven to be a rich source of therapeutic targets; therefore, finding compounds that regulate these receptors is a critical goal in drug discovery. The Transfluor technology utilizes the redistribution of fluorescently labeled arrestins from the cytoplasm to agonist-occupied receptors at the plasma membrane to monitor quantitatively the activation or inactivation of GPCRs. Here, we show that the Transfluor technology can be quantitated on the INCell Analyzer system (INCAS) using the vasopressin V(2) receptor (V(2)R), which binds arrestin with high affinity, and the beta(2)-adrenergic receptor (beta(2)AR), which binds arrestin with low affinity. U2OS cells stably expressing an arrestin-green fluorescent protein conjugate and either the V(2)R or the beta(2)AR were plated in 96-well plastic plates and analyzed by the INCAS at a screening rate of 5 min per plate. Agonist dose-response and antagonist dose-inhibition curves revealed signal-to-background ratios of approximately 25:1 and 8:1 for the V(2)R and beta(2)AR, respectively. EC(50) values agreed closely with K(d) values reported in the literature for the different receptor agonists. In addition, small amounts of arrestin translocation induced by sub-EC(50) doses of agonist were distinguished from the background noise of untreated cells. Furthermore, differences in the magnitude of arrestin translocation distinguished partial agonists from full agonists, and Z' values for these ligands were >0.5. These data show that the Transfluor technology, combined with an automated image analysis system, provides a direct, robust, and universal assay for high throughput screening of known and orphan GPCRs.

Ascidian arrestin (Ci-arr), the origin of the visual and nonvisual arrestins of vertebrate

Arrestin is one of the key proteins for the termination of G protein signaling. Activated G protein-coupled receptors (GPCRs) are specifically phosphorylated by G protein-coupled receptor kinases (GRKs) and then bind to arrestins to preclude the receptor/G protein interaction, resulting in quenching of the following signal transduction. Vertebrates possess two types of arrestin; visual arrestin expressed exclusively in photoreceptor cells in retinae and pineal organs, and beta-arrestin, which is expressed ubiquitously. Unlike visual arrestin, beta-arrestin contains the clathrin-binding domain at the C-terminus, responsible for the agonist-induced internalization of GPCRs. Here, we isolated a novel arrestin gene (Ci-arr) from the primitive chordate, the ascidian Ciona intestinalis larvae. The deduced amino acid sequence suggests that Ci-Arr be closely related to vertebrate arrestins. Interestingly, this arrestin has the feature of both visual and beta-arrestin. Whereas the expression of Ci-arr was restricted to the photoreceptors in the larvae similarly to visual arrestin, the gene product, containing the clathrin-binding domain, promoted the GPCR internalization in HEK293tsA201 cells similarly to beta-arrestin. The phylogenetic tree shows that Ci-Arr is branched from a common root of visual and beta-arrestins. Southern analysis suggests that the Ciona genome contains only one gene for the arrestin family. These results suggest that the visual and beta-arrestin genes were generated by the duplication of the prototypical arrestin gene like Ci-arr in the early evolution of vertebrates.

Apparent loss-of-function mutant GPCRs revealed as constitutively desensitized receptors

The DRY motif is a triplet amino acid sequence (aspartic acid, arginine, and tyrosine) that is highly conserved in G protein-coupled receptors (GPCRs). Recently, we have shown that a molecular determinant for nephrogenic diabetes insipidus, the vasopressin receptor with a substitution at the DRY motif arginine (V2R R137H), is a constitutively desensitized receptor that is unable to couple to G proteins due to its constitutive association with beta-arrestin [Barak, L. S. (2001) Proc. Natl. Acad. Sci. U.S.A. 98, 93-98]. Additionally, the mutant receptors are localized in endocytic vesicles, identical to wild-type receptors stimulated with agonist. In this study, we asked whether the constitutively desensitized phenotype observed in the V2R R137H represents a general paradigm that may be extended to other GPCRs. We show that arginine substitutions in the DRY motifs of the alpha(1B) adrenergic receptor (alpha(1B)-AR) and angiotensin II type 1A receptor (AT(1A)R) result in receptors that are uncoupled from G proteins, associated with beta-arrestins, and found localized in endocytic vesicles rather than at the plasma membrane in the absence of agonists. The localization of the alpha(1B)-ARs and AT(1A)Rs with arginine substitutions can be restored to the plasma membrane by either using selective antagonists or preventing the endocytosis of the beta-arrestin-receptor complexes. These results indicate that the arginine residue of the DRY motif is essential for preserving the localization of the inactive receptor complex. Furthermore, constitutive desensitization may underlie some loss-of-function receptor phenotypes and represent an unappreciated mechanism of hormonal resistance.

Differential nucleocytoplasmic shuttling of beta-arrestins. Characterization of a leucine-rich nuclear export signal in beta-arrestin2

beta-arrestins (betaarrs) are two highly homologous proteins that uncouple G protein-coupled receptors from their cognate G proteins, serve as adaptor molecules linking G protein-coupled receptors to clathrin-coat components (AP-2 complex and clathrin), and act as scaffolding proteins for ERK1/2 and JNK3 cascades. A striking difference between the two betaarrs (betaarr1 and betaarr2) is that betaarr1 is evenly distributed throughout the cell, whereas betaarr2 shows an apparent cytoplasmic localization at steady state. Here, we investigate the molecular determinants underlying this differential distribution. betaarr2 is constitutively excluded from the nucleus by a leptomycin B-sensitive pathway because of the presence of a classical leucine-rich nuclear export signal in its C terminus (L395/L397) that is absent in betaarr1. In addition, using a nuclear import assay in yeast we showed that betaarr2 is actively imported into the nucleus, suggesting that betaarr2 undergoes constitutive nucleocytoplasmic shuttling. In cells expressing betaarr2, JNK3 is mostly cytosolic. A point mutation of the nuclear export signal (L395A) in betaarr2, which was sufficient to redistribute betaarr2 from the cytosol to the nucleus, also caused the nuclear relocalization of JNK3. These data indicate that the nucleocytoplasmic shuttling of betaarr2 controls the subcellular distribution of JNK3.

Seven-transmembrane receptors

Seven-transmembrane receptors, which constitute the largest, most ubiquitous and most versatile family of membrane receptors, are also the most common target of therapeutic drugs. Recent findings indicate that the classical models of G-protein coupling and activation of second-messenger-generating enzymes do not fully explain their remarkably diverse biological actions.

Maturation of respiratory control in the behaving mammal

Whereas in vitro techniques have contributed greatly to our understanding of detailed neuronal mechanisms of respiratory control, the integrated function of respiratory behavior requires studying conscious, unsedated subjects. Noninvasive approaches, meticulous chronic instrumentation for the recording of multiple respiratory indices, and correlations with brain studies performed after physiological manipulations in vivo can all be employed to get to some understanding of the maturation of respiratory control in the mammal. This article is a selective and critical overview of recent literature on methodologies that can be used in behaving subjects, the relationship of respiration to sleep-wake states, respiratory patterns during normoxia, and on respiratory responsiveness to hypercarbia and hypoxia, all emphasizing processes during development. It is hoped that this review will encourage new investigators interested in the regulation of breathing to resort to experimental approaches that will reveal the mysteries of respiratory behavior in the integrated organism.

Phosphorylation of beta-arrestin2 regulates its function in internalization of beta(2)-adrenergic receptors

Beta-arrestins mediate agonist-dependent desensitization and internalization of G protein-coupled receptors. Previously, we have shown that phosphorylation of beta-arrestin1 by ERKs at Ser-412 regulates its association with clathrin and its function in promoting clathrin-mediated internalization of the receptor. In this paper we report that beta-arrestin2 is also phosphorylated, predominantly at residues Thr-383 and Ser-361. Isoproterenol stimulation of the beta(2)-adrenergic receptor promotes dephosphorylation of beta-arrestin2. Mutation of beta-arrestin2 phosphorylation sites to aspartic acid decreases the association of beta-arrestin2 with clathrin, thereby reducing its ability to promote internalization of the beta(2)-adrenergic receptor. Its ability to bind and desensitize the beta(2)-adrenergic receptor is, however, unaltered. These results suggest that, analogous to beta-arrestin1, phosphorylation/dephosphorylation of beta-arrestin2 regulates clathrin-mediated internalization of the beta(2)-adrenergic receptor. In contrast to beta-arrestin1, which is phosphorylated by ERK1 and ERK2, phosphorylation of beta-arrestin2 at Thr-383 is shown to be mediated by casein kinase II. Recently, it has been reported that phosphorylation of visual arrestin at Ser-366 prevents its binding to clathrin. Thus it appears that the function of all arrestin family members in mediating internalization of G protein-coupled receptors is regulated by distinct phosphorylation/dephosphorylation mechanisms.

NK1 receptor fused to beta-arrestin displays a single-component, high-affinity molecular phenotype

Arrestins are cytosolic proteins that, upon stimulation of seven transmembrane (7TM) receptors, terminate signaling by binding to the receptor, displacing the G protein and targeting the receptor to clathrin-coated pits. Fusion of beta-arrestin1 to the C-terminal end of the neurokinin NK1 receptor resulted in a chimeric protein that was expressed to some extent on the cell surface but also accumulated in transferrin-labeled recycling endosomes independently of agonist stimulation. As expected, the fusion protein was almost totally silenced with respect to agonist-induced signaling through the normal Gq/G11 and Gs pathways. The NK1-beta-arrestin1 fusion construct bound nonpeptide antagonists with increased affinity but surprisingly also bound two types of agonists, substance P and neurokinin A, with high, normal affinity. In the wild-type NK1 receptor, neurokinin A (NKA) competes for binding against substance P and especially against antagonists with up to 1000-fold lower apparent affinity than determined in functional assays and in homologous binding assays. When the NK1 receptor was closely fused to G proteins, this phenomenon was eliminated among agonists, but the agonists still competed with low affinity against antagonists. In contrast, in the NK1-beta-arrestin1 fusion protein, all ligands bound with similar affinity independent of the choice of radioligand and with Hill coefficients near unity. We conclude that the NK1 receptor in complex with arrestin is in a high-affinity, stable, agonist-binding form probably best suited to structural analysis and that the receptor can display binding properties that are nearly theoretically ideal when it is forced to complex with only a single intracellular protein partner.

Identification of a short linear sequence present in the C-terminal tail of the rat follitropin receptor that modulates arrestin-3 binding in a phosphorylation-independent fashion

The rat follitropin receptor (rFSHR) is an unusual G protein-coupled receptor in that agonist-induced activation leads to the phosphorylation of the first and third intracellular loops instead of the C-terminal tail. To determine regions of G protein-coupled receptors that affect internalization independently of phosphorylation we examined the effects of truncations of the C-terminal tail of the rFSHR on agonist-induced internalization. Our studies show that progressive truncations of a region flanked by residues 642 and 651 enhance the internalization of human follicle-stimulating hormone (hFSH). Further characterization of a mutant truncated at residue 649 (designated rFSHR-t649) and another mutant in which the 642-651 region was deleted in the context of the full-length rFSHR, designated rFSHR(Delta642-651), showed that both of them internalized hFSH at rates that were 2-3 times faster than rFSHR-wild type (wt). Like rFSHR-wt, however, the internalization of hFSH mediated by rFSHR-t649 and rFSHR(Delta642-651) can be inhibited with dominant-negative mutants of the non-visual arrestins or dynamin. Alanine-scanning mutagenesis of the 642-651 region suggests that the effects on internalization are not mediated by a single residue, however. In an attempt to understand the molecular basis of the enhanced internalization of hFSH mediated by these mutants we used an assay that can be readily used to assess the association of the rFSHR with the arrestin-3 in co-transfected cells. Using this assay we were able to show that, when compared with rFSHR-wt, rFSHR(Delta642-651) displays an approximately 4-fold enhancement in binding affinity for arrestin-3 and an approximately 1.7-fold reduction in maximal arrestin-3 binding capacity. We conclude that a short linear sequence present in the C-terminal tail of the rFSHR (642SATHNFHARK651) that is not phosphorylated limits internalization by lowering the affinity of the rFSHR for the endogenous non-visual arrestins.

Regulation of arrestin-3 phosphorylation by casein kinase II

Arrestins play an important role in regulating the function of G protein-coupled receptors including receptor desensitization, internalization, down-regulation, and signaling via nonreceptor tyrosine kinases and mitogen-activated protein kinases. Previous studies have revealed that arrestins themselves are also subject to regulation. In the present study, we focused on identifying potential mechanisms involved in regulating the function of arrestin-3. Using metabolic labeling, phosphoamino acid analysis, and mutagenesis studies, we found that arrestin-3 is constitutively phosphorylated at Thr-382 and becomes dephosphorylated upon beta(2)-adrenergic receptor activation in COS-1 cells. Casein kinase II (CKII) appears to be the major kinase mediating arrestin-3 phosphorylation, since 1) Thr-382 is contained within a canonical consensus sequence for CKII phosphorylation and 2) wild type arrestin-3 but not a T382A mutant is phosphorylated by CKII in vitro. Functional analysis reveals that mutants mimicking the phosphorylated (T382E) and dephosphorylated (T382A or T382V) states of arrestin-3 promote beta(2)-adrenergic receptor internalization and bind clathrin, beta-adaptin, and Src to comparable levels as wild type arrestin-3. This suggests that the phosphorylation of arrestin-3 does not directly regulate interaction with endocytic (clathrin, beta-adaptin) or signaling (Src) components and is in contrast to arrestin-2, where phosphorylation appears to regulate interaction with clathrin and Src. However, additional analysis reveals that arrestin-3 phosphorylation may regulate formation of a large arrestin-3-containing protein complex. Differences between the regulatory roles of arrestin-2 and -3 phosphorylation may contribute to the different cellular functions of these proteins in G protein-coupled receptor signaling and regulation.

mu-Opioid receptors desensitize less rapidly than delta-opioid receptors due to less efficient activation of arrestin

Receptor desensitization by G-protein receptor kinases (GRK) and arrestins is likely to be an important component underlying the development of tolerance to opioid drugs. Reconstitution of this process in Xenopus oocytes revealed distinct differences in the kinetics of GRK and arrestin regulation of the closely related opioid receptors mu (MOR), delta (DOR), and kappa (KOR). We demonstrated that under identical conditions, GRK and arrestin-dependent desensitization of MOR proceeds dramatically slower than that of DOR. Furthermore, GRK3 phosphorylation sites required for opioid receptor desensitization also greatly differ. The determinants for DOR and KOR desensitization reside in the carboxyl-terminal tail, whereas MOR depends on Thr-180 in the second intracellular loop. Although this later finding might indicate an inefficient phosphorylation of MOR Thr-180, increasing the amount of arrestin expressed greatly increased the rate of MOR desensitization to a rate comparable with that of DOR. Similarly, coexpression of a constitutively active arrestin 2(R169E) with MOR and DOR desensitized both receptors in an agonist-dependent, GRK-independent manner at rates that were indistinguishable. Together, these data suggest that it is the activation of arrestin, rather than its binding, that is the rate-limiting step in MOR desensitization. In addition, mutation of Thr-161 in DOR, homologous to MOR Thr-180, significantly inhibited the faster desensitization of DOR. These results suggest that DOR desensitization involves phosphorylation of both the carboxyl-terminal tail and the second intracellular loop that together leads to a more efficient activation of arrestin and thus faster desensitization.

Differences in the cellular localization and agonist-mediated internalization properties of the alpha(1)-adrenoceptor subtypes

The cellular localization, agonist-mediated internalization, and desensitization properties of the alpha(1)-adrenoceptor (alpha(1)-AR) subtypes conjugated with green fluorescent protein (alpha(1)-AR/GFP) were assessed using real-time imaging of living, transiently transfected human embryonic kidney (HEK) 293 cells. The alpha(1B)-AR/GFP fluorescence was detected predominantly on the cell surface. Stimulation of the alpha(1B)-AR with phenylephrine led to an increase in extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation and promoted rapid alpha(1B)-AR/GFP internalization. Long-term exposure (15 h) to phenylephrine resulted in desensitization of the alpha(1B)-AR-mediated activation of ERK1/2 phosphorylation. Alpha(1A)-AR/GFP fluorescence was detected not only on the cell surface but also intracellularly. The rate of internalization of the cell surface population alpha(1A)-AR/GFPs was slower than that seen for the alpha(1B)-AR. Agonist exposure also resulted in desensitization of the alpha(1A)-AR-mediated increase in ERK1/2 phosphorylation. The alpha(1D)-AR/GFP fluorescence was detected mainly intracellularly, and this localization was unaffected by exposure to phenylephrine. Phenylephrine treatment of alpha(1D)-AR/GFP expressing cells increased ERK1/2 phosphorylation. However, this increase was not significant. Cotransfection with beta-arrestin 1 did not increase the rate or extent of agonist-stimulated alpha(1A)- or alpha(1B)-AR/GFP internalization. However, a dominant-negative form of the beta-arrestin 1, beta-arrestin 1 (319-418), blocked agonist-mediated internalization of both the alpha(1A)- and alpha(1B)-ARs. These data show that transfected alpha(1)-AR/GFP fusion proteins are functional, that there are differences in the cellular distribution and agonist-mediated internalization between the alpha(1)-ARs, and that agonist-mediated alpha(1)-AR internalization is dependent on arrestins and can be desensitized by long-term exposure to an agonist. These differences could contribute to the diversity in physiologic responses regulated by the alpha(1)-ARs.

Arrestin2 and arrestin3 are differentially expressed in the rat brain during postnatal development

Arrestins are adaptor proteins involved in homologous desensitization and trafficking of G protein-coupled receptors. Arrestins bind to activated phosphorylated receptors thus precluding further signal transduction. Two subtypes of non-visual arrestins, arrestin2 and arrestin3, have been cloned. Recently, specificity of various receptors to arrestins and differences in kinetics of receptor desensitization mediated by arrestins have been demonstrated. Both arrestins are expressed in the rat brain. However, quantitative assessment of their expression and detailed distribution are lacking. Here, we used quantitative ribonuclease protection assay and western blot to measure arrestin2 and arrestin3 mRNA and protein in the rat brain during postnatal development. In situ hybridization histochemistry was employed to study the detailed distribution of arrestin mRNAs in the adult and developing brain. Both arrestins were expressed from birth in all regions studied. Arrestin2 mRNA levels increased with development until the 14th postnatal day and then decreased, whereas arrestin2 protein levels continued to rise. Arrestin3 mRNA was maximal in neonates and then decreased, while arrestin3 protein changed little. In newborns and adults, the concentration of arrestin2 mRNA was two- to three-fold higher than that of arrestin3. In neonates, the excess of the arrestin2 protein over arrestin3 was commensurate with the excess of the arrestin2 mRNA (three-fold) but in the adult, the ratio was much higher (10-20-fold). Each arrestin demonstrated a unique distribution, although in many areas there was overlap suggesting co-localization. Both arrestins were highly expressed in the cortex and hippocampus. Arrestin2 was abundant in the thalamus, particularly in the anterior, intralaminar, and midline nuclei, while arrestin3 was abundant in the medial habenular. Arrestin3 was relatively abundant in most hypothalamic nuclei and extended amygdala. In the developing brain, arrestin3 was highly expressed in the subventricular zone, whereas arrestin2 was more abundant in differentiated areas. Our data demonstrate that arrestin2 is the major arrestin subtype in the rat brain, although arrestin3 is expressed in specific cell populations including postnatal proliferative zones. Because each arrestin appears to mediate receptor desensitization in a specific way, different kinetics of trafficking of the same receptor should be expected in different cells due to varying arrestin2/arrestin3 ratios. Thus, the response of receptors to specific drugs stimulating or blocking these receptors may depend on complement of arrestins in their target cells.

beta-Arrestin scaffolding of the ERK cascade enhances cytosolic ERK activity but inhibits ERK-mediated transcription following angiotensin AT1a receptor stimulation

beta-Arrestins are cytosolic proteins that mediate homologous desensitization of G protein-coupled receptors (GPCRs) by binding to agonist-occupied receptors and by uncoupling them from heterotrimeric G proteins. The recent finding that beta-arrestins bind to some mitogen-activated protein (MAP) kinases has suggested that they might also function as scaffolds for GPCR-stimulated MAP kinase activation. To define the role of beta-arrestins in the regulation of ERK MAP kinases, we examined the effect of beta-arrestin overexpression on ERK1/2 activation and nuclear signaling in COS-7 cells expressing angiotensin II type 1a receptors (AT1aRs). Expression of either beta-arrestin1 or beta-arrestin2 reduced angiotensin-stimulated phosphatidylinositol hydrolysis but paradoxically increased angiotensin-stimulated ERK1/2 phosphorylation. The increase in ERK1/2 phosphorylation in beta-arrestin-expressing cells correlated with activation of a beta-arrestin-bound pool of ERK2. The beta-arrestin-dependent increase in ERK1/2 phosphorylation was accompanied by a significant reduction in ERK1/2-mediated, Elk1-driven transcription of a luciferase reporter. Analysis of the cellular distribution of phospho-ERK1/2 by confocal immunofluorescence microscopy and cellular fractionation revealed that overexpression of beta-arrestin resulted in a significant increase in the cytosolic pool of phospho-ERK1/2 and a corresponding decrease in the nuclear pool of phospho-ERK1/2 following angiotensin stimulation. beta-Arrestin overexpression resulted in formation of a cytoplasmic pool of beta-arrestin-bound phospho-ERK, decreased nuclear translocation of phospho-ERK1/2, and inhibition of Elk1-driven luciferase transcription even when ERK1/2 was activated by overexpression of cRaf-1 in the absence of AT1aR stimulation. These data demonstrate that beta-arrestins facilitate GPCR-mediated ERK activation but inhibit ERK-dependent transcription by binding to phospho-ERK1/2, leading to its retention in the cytosol.

Heterologous regulation of trafficking and signaling of G protein-coupled receptors: beta-arrestin-dependent interactions between neurokinin receptors

Cells express multiple G protein-coupled receptors that are simultaneously or sequentially activated by agonists. The consequences of activating one receptor on signaling and trafficking of another receptor are unknown. We examined the effects of selective activation of the neurokinin 1 receptor (NK1R) on signaling and trafficking of the NK3R and vice versa. Selective agonists of NK1R and NK3R induced membrane translocation of beta-arrestins (beta-ARRs). Dominant negative beta-ARR(319-418) inhibited endocytosis of NK1R and NK3R. Whereas an NK1R agonist caused sequestration of NK1R with beta-ARR in the same endosomes, thereby depleting them from the cytosol, beta-ARRs did not prominently sequester with the activated NK3R and rapidly returned to the cytosol. In cells coexpressing both receptors, prior activation of the NK1R inhibited endocytosis and homologous desensitization of the NK3R, which was dose-dependently reversed by overexpression of beta-ARR1. Similar results were obtained in enteric neurons that naturally coexpress the NK1R and NK3R. In contrast, activation of the NK3R did not affect NK1R endocytosis or desensitization. Thus, the high-affinity and prolonged interaction of the NK1R with beta-ARRs depletes beta-ARRs from the cytosol and limits their role in desensitization and endocytosis of the NK3R. Because beta-ARRs are critical for desensitization, endocytosis, and mitogenic signaling of many receptors, this sequestration is likely to have important and widespread implications.